The Acoustics of Eavesdropping

Where not to have a private conversation.

In 1824, naval officer Edward Boid described how a curve can dramatically amplify sound, and not always for the best. He wrote, “In the Cathedral of Girgenti, in Sicily, the slightest whisper is borne with perfect distinctness from the great western door to the cornice behind the high altar—a distance of two hundred and fifty feet.” Unfortunately, the confessional was badly sited: “Secrets never intended for the public ear thus became known, to the dismay of the confessors, and the scandal of the people ... ​till at length, one listener having had his curiosity somewhat over-gratified by hearing his wife’s avowal of her own infidelity, this tell-tale peculiarity became generally known, and the confessional was removed.”

For centuries, people have known that curved surfaces amplify sounds and allow covert listening. Athanasius Kircher, who wrote extensively on echoes, gave a good explanation in the 17th century. His publications also document some fantastical devices, including giant ear trumpets built into the walls of royal chambers for eavesdropping. Probably his most famous—or infamous—device is the Katzenklavier (literally, “cat piano”). It has a normal piano keyboard in front of a line of cages, each of which has a cat trapped inside. Every time a piano key is pressed, a nail is driven into the tail of one unfortunate feline, which naturally screeches. With the right set of cats, ones that shriek at different frequencies, a sadistic musician could play a tune on the instrument.

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The sound would have been excruciating, but then it was designed to shock psychiatric patients into changing their behavior, rather than being a genuine instrument for playing Monteverdi or Purcell. Fortunately, it is unlikely that it was ever built.

At this point you might be doubting the sanity and rationality of Kircher. Yet he drew diagrams that illustrated a good scientific understanding of how an elliptical ceiling can enhance communication between two people.

The lines in the diagram show the paths that sound “rays” take when going from the speaker to the listener. These ray paths can be worked out using a ruler and protractor. Alternatively, by treating the room as a weird-shaped pool table, the paths can be worked out by following the line a cue ball would take (ignoring gravity). If the cue ball is placed at the speaker’s mouth and fired toward the ceiling, it will always go to the listener. So all the sound going upward is focused at the listener, allowing even quiet whispers to be heard across a large room.

The problem with Kircher’s design is that the listener and speaker have to stand in particular places—the foci of the ceiling ellipse. The design is not very useful if one person wants to talk to an audience of listeners scattered around the room.

A few years ago I presented two science shows at the Royal Albert Hall in London to thousands of children. Though better known as a music venue, the hall is actually dedicated to the promotion of art and science, and it was built on land purchased with the profits of the Great Exhibition of 1851. Fortunately, the acoustics have been significantly improved since the hall opened 130 years ago. Indeed, the Prince of Wales struggled with his opening speech. According to the Times of London in 1871:

The address was slowly and distinctly read by his royal Highness, but the reading was somewhat marred by an echo which seemed to be suddenly awoke from the organ or picture gallery, and repeated the words with a mocking emphasis which at another time would have been amusing.

The hall’s ubiquitous curved surfaces are probably what caused the mocking echoes. From above, the floor plan appears as an ellipse, and the whole structure is topped with a large dome. The curved surfaces focus sound like Kircher’s elliptical ceiling. But how such reflections are perceived depends on the size of the room. In the vast Royal Albert Hall, the curves cause disastrous echoes. Sound appears to come from several places in the room and not just the stage. In a small room the focused sound arrives quickly; in a larger room the reflections are delayed.

The way the brain combines sounds is important, because otherwise we would rapidly become overwhelmed by the vast number of reflections that accompany us. As I type this sentence, the rattle of the keyboard is being reflected off the desk, the computer monitor, my phone, the ceiling, and so on. Yet my hearing is not overwhelmed by all these different reflections; the sound still appears to be coming straight from the keyboard as it should be.

The same thing happens in Kircher’s small room. The reflections from the elliptical ceiling arrive quite quickly, and unless the reflections are very loud, the brain does not hear them as separate from sound traveling directly between talker and listener. By contrast, the Royal Albert Hall is so vast that the focused reflections arrive much later, creating “mocking” echoes.

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The renovation of the U.S. Capitol in the 19th century in Washington ruined a fine ceiling echo from a famous whispering dome. The Capitol’s dome used to be an almost perfect hemisphere centered at the head height of visitors, and although the ceiling appeared coffered with indented squares, it was actually smooth, with trompe l’oeil painting creating the illusion of structure and texture. Before 1901, this domed space was a great draw for tourists. According to the New York Times in 1894:

The whispering gallery still holds the palm among the show places of the great marble structure. Once in a while an old resident of Washington is initiated into the mysteries of the echoes and other acoustic phenomena which abound in this old-time chamber, and he feels a little ashamed of his tardiness in seeking this remarkable entertainment.

But while it was great fun for the tourists, it was a poor place for the House of Representatives to hold debates. As Maine’s Lewiston Daily Sun put it in 1893:

The orator who was not cautious enough to remain in one spot during the delivery of his address found the acoustics of the hall taking strange liberties with his elocution, transforming his crescendo sentences into comical squeals, or causing his pianissimo phrases, his stage whispers, to shriek and wail as he moved to and fro from one echo point to another.

A gas explosion and fire in 1898 elsewhere in the building led to replacement of the wooden dome by a fireproof construction. Real plaster coffers were installed in place of the trompe l’oeil—changing from a smooth surface to one covered in lumps and bumps—making the effect of the focus weaker and less remarkable.

While domes are fun, a completely spherical room is even better because reflections are amplified even more.

The Mapparium in Boston is a 30-foot sphere and was built in 1935 following a suggestion by architect Chester Lindsay Churchill. It is a giant hollow globe of the world, with the seas and continents vividly drawn on stained glass. Visitors traverse a walkway cutting through the center of the Earth linking up two opposite points on the equator. Three hundred light bulbs illuminate the globe from the outside. Looking at the world from the inside out is an odd experience, but what also strikes visitors are the strange acoustics, which were an accidental byproduct of the geometry.

William Hartmann, from Michigan State University, and colleagues documented the various illusions that can be heard. Usually, moving farther away from a listener makes a talker’s voice quieter, but that’s not always the case in a spherical room. Imagine, Hartmann writes, “you are on the Mapparium bridge 7 feet to the left of dead center. Your friend is exactly at the center and is talking to you. His voice seems rather quiet. Now your friend walks away from you, and his voice gets louder and louder until he is about [7 feet] to the right of dead center.”

Courtesy of Trevor Cox

The sketches at left show what is happening (to make it easier to see, this drawing uses a circle rather than a complete sphere). When the talker speaks from the center (top diagram), all the reflections are focused back, so the talker appears to be surprisingly quiet to the listener left of center. If the talker moves to the right, then the place where the reflections focus moves closer to the listener. The sound will be loudest when the talker and listener are arranged symmetrically about the center (bottom diagram). The unusually strong focus makes it possible to experience the strange sensation of whispering into your own ear. Or, as Hartmann puts it: “As you approach the exact center of the Mapparium sphere you suddenly become aware of strong reflections of your own voice. ... ​If you sway to the left, you hear yourself in your right ear. If you sway to the right, you hear yourself in your left ear.”

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Whispering walls do not focus sound as happens with elliptical ceilings and domes; the listener and speaker are too far from the focal point of the arc. Instead, the sound hugs the inside of the concrete wall and is transported with surprising loudness to the other side.

The largest real whispering wall I know of is 460 feet long—it is the concrete dam that withstands the Barossa Reservoir in South Australia. For some reason the dam was built to be a precise arc. This vast, gray slab of concrete has turned into an unlikely tourist attraction, with visitors chatting with each other from opposite ends of the dam.

Whispering arches behave in a similar way, and they also show up in the most unlikely of places. On the lower level of Grand Central Terminal in New York City, outside the famous Oyster Bar & Restaurant, sweeping tiled archways, designed by Rafael Guastavino and his son in 1913, support the ceiling. If you whisper into one side of the arch, the sound follows the curve of the tiled ceiling before coming back down the other side. For the best effect, the whisperer and listener need to get close to the stone, like naughty children standing in opposite corners of a classroom.

The Grand Terminal Oyster Bar arch in Grand Central Station in New York.

The sound effect has inspired literature and films; the author Katherine Marsh uses the whispering arches as the starting point in her children’s books The Night Tourist and The Twilight Prisoner, describing the arches as “one of the coolest places in New York.”

The delight in these places comes from hearing a voice carry an unexpected distance, and this effect is more dramatic if the sound is a quiet whisper to begin with. Mathematical analysis by Lord Rayleigh, the Nobel Prize–winning physicist and author of the 19th-century acoustic bible The Theory of Sound, suggests another reason for whispering: High frequencies, like the sibilant tones in whispers, hug the walls closer than the lower-frequency sounds of normal speech.